Systematic Studies Of The Formation And Diffusion Of Oxygen Vacancy In HfO₂-based Ferroelectrics By Ab Initio Simulations

As a new type of ferroelectric materials,hafnium oxides with a ferroelectric orthorhombic phase（ferroelectric hafnium oxides）show great development potential in the application of non-volatile ferroelectric memory.However,there are still a series of reliability problems in ferroelectric hafnium oxides,such as wake-up effect and fatigue failure,which restrict the further development of ferroelectric hafnium oxides.According to previous studies,oxygen vacancies and element doping may be the key to solving these reliability problems.However,current research on oxygen vacancy and element doping mainly focuses on how to stabilize the ferroelectric phase of hafnium oxides,while ignoring the formation mechanism of oxygen vacancies and the effect of element doping on the formation and diffusion of oxygen vacancies.Therefore,in this thesis,the formation energy and concentration of oxygen vacancies in hafnium ferroelectric oxides and La doped hafnium ferroelectric oxides under different oxygen content are systematically studied by first-principles calculation method,and the influencing factors of diffusion barrier of oxygen vacancy are also explored.The main research contents of this thesis are as follows:（1）The effect of oxygen vacancies on the electronic structure and polarization properties of ferroelectric hafnium oxides was studied.In ferroelectric hafnium oxides,the valence band near the Fermi level is mainly contributed by O2p orbitals,which exist as bonding orbitals when Hf atoms and O atoms form Hf-O bonds.In addition,when the 3-coordinate oxygen vacancy carries two positive charges,the polarization of ferroelectric hafnium oxides decreases by about 19μC/cm².（2）The effect of oxygen content on the defect formation energy of ferroelectric hafnium oxides and La doped hafnium ferroelectric oxides was studied.Under the O-poor condition,the oxygen vacancies have a low forming energy in ferroelectric hafnium oxides,thus stabilizing the ferroelectric phase well,where the concentration of neutral oxygen vacancies is about 1.1×10¹⁶ cm^-3.La doping induces the transformation of the neutral oxygen vacancies to the doubly positively charged oxygen vacancies,which is conducive to improving the fatigue properties of the ferroelectric hafnium oxides,and reduces the formation energy for oxygen vacancies under different oxygen content.（3）The effect of La doping on the diffusion barrier of oxygen vacancies in ferroelectric hafnium oxides was studied.It was found that the La defect obstructs the diffusion of the doubly positively charged oxygen vacancies in ferroelectric hafnium oxides,which leads to the enhancement of the wake-up effect in ferroelectric hafnium oxides.The diffusion barrier of the neutral oxygen vacancies in ferroelectric hafnium oxides is about 2.41 e V,while the doubly positively charged oxygen vacancies is only about 0.43 e V,indicating that the doubly positively charged oxygen vacancies are more prone to diffusion.The diffusion barrier of the neutral oxygen vacancies in La doped ferroelectric hafnium oxides decreases to about 1.96 e V,while the diffusion barrier of the doubly positively charged oxygen vacancies increases to about 1.02 eV.